This invention relates to engine catalyst monitoring system and methods.
As is known in the art, Government regulations place strict emissions standards on automobiles. Furthermore, to ensure low emissions throughout vehicle life, automobiles must be self-diagnosing; i.e., failures in any emissions related component must be reported to the driver by illuminating a malfunction indicator light (MIL), for example. The catalytic converter (herein referred to as the catalyst), being one of the most critical emission control components on an automobile, falls under these rules. The ability of the catalyst to convert emissions into benign compounds must be monitored during engine operation.
Traditionally, three way catalysts (TWCs) are diagnosed based on oxygen storage since it correlates with hydrocarbon (HC) and NOx , conversion efficiency. To measure oxygen storage, oxygen sensors are used to detect the oxygen concentration upstream and downstream of the catalyst. By analyzing the differences in the upstream and downstream concentrations of oxygen, the oxygen storage of the catalyst, and therefore its conversion efficiency, can be inferred.
A variety of methods have been developed and patented to analyze the oxygen sensor signals for catalyst diagnosis. Features of the sensor signals are converted into metrics, and diagnosis is performed using these metrics. Some metrics include number of rich/lean switches, amplitudes, slopes, length of line, and step responses. As vehicle emissions levels proceed from Low Emission Vehicle (LEV) to Ultra-low Emission Vehicle (ULEV) to Super Ultr-Low Emission Vehicle/Partial Zero Emission Vehicle (SULEV/PZEV), however, all these metrics have increased difficulty distinguishing between good and failed TWCs, leading to unnecessary warranty costs. Furthermore, new types of TWC materials and designsxe2x80x94conditioning catalysts and low oxygen storage catalysts, for examplexe2x80x94create challenges for existing catalyst monitor techniques.
In summary, the automotive industry faces the problem of diagnosing new types of catalysts with more accuracy using modest computing resources and new technology has been created to meet this challenge.
In accordance with the present invention, a method is provided for determining a condition of a catalyst disposed in an exhaust of an engine. The method includes sensing a common property of both the exhaust upstream and downstream of the catalyst. Samples of such upstream and downstream sensed property are taken over a period of time. The taken samples are accumulated over the period of time. Statistical characteristics of the sensed common property of the upstream and downstream common property are determined. The determined statistical characteristics of the sensed upstream property are compared with the determined statistical characteristics of the downstream property to determine whether catalyst was in a proper operating condition during such period of time.
With such method statistical characteristics are determined from samples as they are obtained and then once obtained, processed to determine the condition of the catalyst. Such method thereby reduces memory or data storage requirements and also reduces computational requirements.
In one embodiment, the common property is oxygen content in the exhaust.
In one embodiment one of the statistical characteristics is the mean of the samples of the upstream property and the mean of the downstream property.
In one embodiment another one of the statistical characteristics is the mean of the square of the samples of the upstream property and the square of the mean of the downstream property.
In one embodiment, another one of the statistical characteristic is the variance of the samples of the upstream property and the variance of the samples of the downstream property.
In one embodiment, the variance of the samples of the upstream property is determined by calculating the average of the squares of such upstream samples minus the square of the mean of such upstream samples and wherein the variance of the samples of the downstream property is determined by calculating the average of the squares of such downstream samples minus the square of the mean of such downstream samples.
In accordance with the invention, a method is provided for determining a condition of a catalyst disposed in an exhaust of an engine. The method includes providing oxygen sensors in the exhaust upstream and downstream of the catalyst; estimating the mean value of the upstream and downstream sensor signals, mf and mr respectively, in accordance with:             m      f        =                            1          N                ⁢                              ∑                          t              =              1                        N                    ⁢                      xe2x80x83                    ⁢                                                    v                f                            ⁡                              (                t                )                                      ⁢                          xe2x80x83                        ⁢            and            ⁢                          xe2x80x83                        ⁢                          m              r                                          =                        1          N                ⁢                              ∑                          t              =              1                        N                    ⁢                      xe2x80x83                    ⁢                                    v              r                        ⁡                          (              t              )                                            ,      xe2x80x83    ⁢  respectively
where vf(t) and vr(t) are the voltages of the front and rear oxygen sensor respectively at time t, and N is the number of samples over the period of time; estimating the variances and correlation coefficient, denoted by sf2, sr2, and r respectively, in accordance with:       s    f    2    =                              1          N                ⁢                              ∑                          t              =              1                        N                    ⁢                      xe2x80x83                    ⁢                      (                                          (                                                      v                    f                                    ⁢                                      (                    t                    )                                                  )                            2                        )                              -                        (                                    1              N                        ⁢                                          ∑                                  t                  =                  1                                N                            ⁢                              xe2x80x83                            ⁢                                                v                  f                                ⁢                                  (                  t                  )                                                              )                          2          ⁢                      
                                ⁢          
        ⁢                  s        r        2            =                                                  1              N                        ⁢                                          ∑                                  t                  =                  1                                N                            ⁢                              xe2x80x83                            ⁢                              (                                                      (                                                                  v                        r                                            ⁢                                              (                        t                        )                                                              )                                    2                                )                                              -                                    (                                                1                  N                                ⁢                                                      ∑                                          t                      =                      1                                        N                                    ⁢                                      xe2x80x83                                    ⁢                                                            v                      r                                        ⁢                                          (                      t                      )                                                                                  )                                      2              ⁢                              
                                                    ⁢                  
                ⁢                                            rs              f                        ⁢                          s              r                                =                                                    1                N                            ⁢                                                ∑                                      t                    =                    1                                    N                                ⁢                                  xe2x80x83                                ⁢                                                                            v                      f                                        ⁢                                          (                                              t                        -                        T                                            )                                                        ⁢                                                            v                      r                                        ⁢                                          (                      t                      )                                                                                            -                                          (                                                      1                    N                                    ⁢                                                            ∑                                              t                        =                        1                                            N                                        ⁢                                          xe2x80x83                                        ⁢                                                                  v                        f                                            ⁢                                              (                                                  t                          -                          T                                                )                                                                                            )                            ⁢                              (                                                      1                    N                                    ⁢                                                            ∑                                              t                        =                        1                                            N                                        ⁢                                          xe2x80x83                                        ⁢                                                                  v                        r                                            ⁢                                              (                        t                        )                                                                                            )                                                        
where T is a positive constant; and determining the condition of the catalyst by comparing the differences between the estimated means of the upstream and downstream sensor signals, the difference in the estimated variances of the upstream and downstream sensor signals, and the estimated correlation coefficient.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.